Nanoscale features revealed by a multiscale characterization of discordant monazite highlight mobility mechanisms of Th and Pb
Résumé
Understanding radionuclides mass transfer mechanisms in monazite (LREEPO4) and the resulting features, from the micro-to the nanoscale, is critical to its use as a robust U-Th-Pb geochronometer. A detailed multi-scale characterization of discordant monazite grains from a granulite which records a polymetamorphic history, explores the mechanisms of Th and Pb mobility in crystals. Some monazite grains display Th-rich linear features (0.1-1 µm thick) associated with a chemically varied suite of amorphous silicate (±Al, Mg, Fe) phases or sulphur (e.g. FeS). They are interpreted as precipitates within monazite crystals. They formed during replacement mechanism of monazite through fluid interactions. Two generations of Pb*bearing nanophases exist supported by previous geochronological data. The shielding effect of garnet and rutilated quartz (host minerals), limiting fluid access, induces plentiful Pb*-bearing nanophases precipitation (fluid saturation enhanced) and limits Pb*-loss at the grainscale. This multi-scale study provides new insights for interpretations of meaningless geochronological information thanks to nanoscale investigations.
| Origine | Fichiers produits par l'(les) auteur(s) |
|---|---|
| Licence |
